The present invention relates to chemical mechanical planarization (CMP) techniques and, more particularly, to a method for applying downward force on a wafer during CMP and an apparatus for applying a wafer to a polishing surface during a CMP operation.
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess material.
A chemical mechanical planarization (CMP) system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically made of a polyurethane material or polyurethane in conjunction with other materials such as, for example a stainless steel belt. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10 which is typically utilized in a CMP system. The linear polishing apparatus 10 polishes away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer typically includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum and copper), metal alloys, semiconductor materials, etc. Typically, CMP may be utilized to polish the one or more of the layers on the wafer 16 to planarize a surface layer of the wafer 16.
The linear polishing apparatus 10 utilizes a polishing belt 12, which moves linearly in respect to the surface of the wafer 16. The belt 12 is a continuous belt rotating about rollers 20. The rollers are typically driven by a motor so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a linear motion 22 with respect to the wafer 16.
The wafer 16 is held by a polishing head 18. The wafer 16 is typically held in position by mechanical retaining ring and/or by vacuum. The polishing head 18 positions the wafer atop the polishing belt 12 and moves the wafer 16 down to the polishing belt 12. The polishing head 18 applies the wafer 16 to the polishing belt 12 with pressure so that the surface of the wafer 16 is polished by a surface of the polishing belt 12. The polishing head 18 is typically part of a spindle drive assembly 30 (shown in FIG. 1B) that enables application of polishing pressure to the wafer 16.
FIG. 1B shows a conventional spindle drive assembly 30 that may be utilized to apply the wafer 16 to the polishing belt in the CMP apparatus 10 (as shown above in FIG. 1A). The spindle drive assembly 30 includes the polishing head 18 connected to a spindle 42. The spindle 42 is attached to a force magnifier 34 that in one end is connected to a hinge 40 and in the other end is connected to an air cylinder 32. The force magnifier 34 is typically an a machined aluminum arm that acts in a similar manner to a lever so force applied by the air cylinder 32 is magnified onto the spindle 42. The spindle 42 then pushes down the polishing head 18 which in turn applies pressure to the wafer 16 for polishing action (as shown in FIG. 1A).
Generally, a range of 3 psi to 10 psi can be applied to the wafer 16 by the spindle drive assembly 30. Unfortunately, at pressures lower than 3 psi, the by the spindle drive assembly 30 is unable to apply a consistent, controlled pressure. The air cylinder 32 is typically controlled with a pneumatic servo valve that uses feedback from a load cell 36 inside the polishing head 18. Problematically the weight of the spindle, head, and other hardware is not supported by anything other than the spindle. This makes the application of downward forces lower than the weight attached to the cylinder 32 very unstable. Also, because of the force magnifier 34, small adjustments in pressure made at the cylinder 32 cause large pressure application changes in the polishing head 18 so control of pressure is very difficult. In certain circumstances, the inability to control low force application prevents a gentle touchdown of the wafer onto the polishing pad. This often occurs because of an inherent overshoot built into the spindle drive assembly 30 for a particular pressure setting. For example, if pressure of 4 psi is desired to be applied to the wafer, a pressure of 5 psi is generally applied to break friction within individual components of the spindle drive assembly 30 and move the spindle. Therefore, low polishing pressure application to the wafer using conventional pressure application systems is very problematic.
Additionally, because of the indirect linkage of air cylinder 32 to the rest of the spindle drive assembly 30, reduced stability of the polishing head 18 often occurs. Therefore, consistent polishing pressure on a wafer, especially at low pressure levels is often difficult to attain.
Therefore, there is a need for an apparatus that overcomes the problems of the prior art by having a downward force application apparatus that can optimize control of polishing pressure applied by a polishing head to a wafer in CMP systems.
Broadly speaking, the present invention fills this need by enabling the optimal control of downward force application in a chemical mechanical planarization (CMP) polishing process. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In accordance with one aspect of the invention, an apparatus for applying a wafer to a polishing surface during a CMP operation is provided. The apparatus includes a spindle that has an upper end and a lower end. A wafer carrier is coupled to the lower end of the spindle. A linear force generator is disposed at the upper end of the spindle. A load cell is positioned between the linear force generator and the upper end of the spindle. A controller is coupled to the load cell for controlling the force applied by the linear force generator.
In one embodiment, the linear force generator includes a lower plate that is disposed on the load cell and an upper plate supported above the lower plate. The linear force generator also includes a bladder positioned between the lower plate and the upper plate. In another embodiment, a load cell plate is coupled to the upper end of the spindle, and a load cell is disposed on the load cell plate.
In accordance with another aspect of the invention, a method for applying downward force on a wafer during chemical mechanical planarization (CMP) is disclosed. In this method, a linear downward force is applied to an upper end of a spindle. The spindle has a wafer carrier coupled to a lower end thereof. The method also monitors the linear downward force applied on the upper end of the spindle.
The advantages of the present invention are numerous. Most notably, by creating an apparatus that is configured to optimally control and apply linear downward force onto a wafer, control over polishing pressures utilized in CMP may be significantly improved. Specifically, a force generation assembly may be connected to an upper end of the spindle, and the lower end of the spindle may be connected to a wafer carrier. This structure enables direct linear application of force to a wafer. In this way, the range of consistent force application may be expanded and low force application to the wafer can be enhanced. In addition, the force application apparatus described herein augments wafer carrier stability which even further optimizes wafer processing. Consequently, the force application apparatus enables highly advantageous wafer polishing pressure control and improved wafer processing efficiency.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.